Among the small format printing sheets, a computer card or name card has a uniform thickness such that, when a number of these cards are stacked in a pile, each sheet of the pile will always remain parallel to the horizontal, and can be readily picked up by a plurality of suction cups disposed above the pile. On the other hand, an envelope has a non-uniform thickness, with the flap thereof representing a thicker area as compared to the remaining portion of the envelope. Thus, as a number of envelopes are stacked in a pile, each envelope of the pile will be inclined progressively relative to the horizontal so that the envelopes can no longer be picked up and supplied to the printing machine. Heretofore, wedge-shaped inserts had to be introduced by an operator into a space between the sheet edges at intervals of say fifty sheets for holding the uppermost sheet of the pile in a nearly horizontal position. These inserts had to be removed by the operator as the sheets are continuously fed to the printing machine. This is an extremely tiresome operation and gives rise to increased labor costs.
For example, the U.S. Pat. No. 3,446,500 shows an automatic device designed for offsetting the non-uniform thickness of printed sheets. The device comprises a bottom plate pivotally connected to the hopper and depressed under the weight of the pile which is opposed by the force of resilient means associated with the bottom plate. As the sheets are decreased in the course of the printing operation, the opposing force exerted by such resilient means is gradually reduced for maintaining the upper surface of the pile substantially parallel with respect to the horizontal.
This known device has however a drawback that the spring force may not always be correctly adjusted in advance and hence the horizontal position of the paper sheet can not be attained satisfactorily.
Moreover, in printing paper sheets of a small size or format, a special printing machine had to be employed for printing of small format sheets, and there has been a strong demand for a paper sheet feeding attachment that may be readily attached to and removed from the existing printing machine used for printing of regular size sheets.
These two problems can be solved simultaneously by the feeding device or attachment of the present invention.